The present invention relates to automatic transmissions, and more particularly to an eight-speed automatic transmission for use in a motor vehicle.
A typical automatic transmission in a motor vehicle has two or three planetary gear sets, one of which receives a torque input from an engine, another one of which is coupled to a drive shaft for providing a torque output. During operation, a set of frictional units couple the torque input from the engine to one or more members of the planetary gear sets. Simultaneously, another set of frictional units holds stationary one or more members of the planetary gear sets. These frictional units provide different ratios of input-to-output torque to the vehicle. It is desirable to provide an automatic transmission for a vehicle that includes a wider range and a larger number of transmission ratios.
Automatic transmissions are typically controlled by a hydraulic control system. These hydraulic control systems are used to engage and disengage the frictional units of the transmission according to the ratio of torque needed. A typical hydraulic control system is disclosed in U.S. Pat. No. 6,159,124 to Redinger et al., herein incorporated by reference. The typical hydraulic control system is composed of various valves that direct and regulate hydraulic pressure to the frictional units via various fluid passages.
The eight-speed automatic transmission according to the principles of the present invention has a compound planetary gear set, a second planetary gear set, and a third planetary gear set. The compound planetary gear set has a small sun gear, a large sun gear, a first carrier, and a first ring gear. The second planetary gear set has a second sun gear drivingly engaged with the first carrier, a second carrier drivingly engaged with the large sun gear, and a second ring gear. The third planetary gear set has a third sun gear, a third carrier drivingly engaged with the second ring gear and drivingly engaged with an output shaft, and a third ring gear drivingly engaged with the second carrier.
The eight-speed automatic transmission further includes a first clutch selectively receiving an input from an engine and drivingly engaged with the third sun gear, a second clutch selectively receiving an input from the engine and drivingly engaged with the large sun gear and the second carrier, and a third clutch selectively receiving an input from the engine and drivingly engaged with the small sun gear. A first brake is engaged with the third ring gear and the second carrier for selectively fixing the third ring gear and the second carrier from rotation. A second brake is engaged with the first ring gear for selectively fixing the first ring gear from rotation. A third brake is engaged with the first carrier for selectively fixing the first carrier from rotation. A fourth brake is engaged with the small sun gear for selectively fixing the small sun gear from rotation. Selectively engaging one of the clutches and one of the brakes provides an input-to-output torque ratio corresponding to one of eight forward gear speeds and at least one reverse speed.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.